Generally, from the standpoint of enhanced working efficiency, there are known electric power tools with a speed changing function (see, e.g., Japanese Patent Laid-open Application No. S63-101545).
One example of the electric power tools is illustrated in FIG. 16. The electric power tool of this example includes a motor 101 as a driving power source, a reduction mechanism 102 for transferring the rotating power of the motor 101, a driving unit (not shown) for transferring the rotating power of the reduction mechanism 102 to a tip end tool, a resin-made housing 104 for containing the motor 101 and the reduction mechanism 102 in a main body portion thereof, the housing 104 provided with a handle portion 104a, a speed changing unit 105 and 105a for changing the reduction ratio of the reduction mechanism 102, the speed changing unit 105 being arranged in such a position as to allow an operator to operate the same from the outside of the housing 104, a power switch 106 for turning on and off the power source of the motor 101 held within the handle portion 104a, and a battery pack 107 engaged with the housing 104 for supplying electric power to the motor 101.
The speed changing unit 105 is kept in a low-speed high-torque state as illustrated in FIG. 17A when the load is high (that is, when the working load is heavy) but is switched to a high-speed low-torque state as illustrated in FIG. 17B when the load is low (that is, when the working load is light). This makes it possible to perform the desired tightening operation depending on the working load, thereby enhancing the working efficiency.
In case where the working load varies in the course of work, the speed changing unit 105 may be switched to change the reduction ratio during the work. However, this may cause trouble to the electric power tool. Specifically, if the reduction ratio is changed during work by use of the speed changing unit 105, the rotating gears 102a of the reduction mechanism 102 are shifted, at which time the gears 102a make contact with each other during their rotation and undergo wear or damage. This may be a cause of trouble in the electric power tool. Conventionally, such trouble is prevented by increasing the strength of the gears 102a. In this case, the gears are formed of high strength metal components or formed into a big size, which entails a drawback of high cost and increased weight.